The present invention relates generally to a method and apparatus for deposition of reagents, and more particularly to an arc plasma deposition method and apparatus in which the reagents are evaporated into the plasma.
The use of a polycarbonate (PC) sheet or film for outdoor applications such as architectural glazing and automotive glazing typically requires the PC to be protected from the ultraviolet (UV) radiation of the sun. PC windows can turn yellow after being exposed to the UV radiation in sunlight. The yellow windows suffer from poor transmission of visible light, thus making them unsuitable for architectural or car window applications.
Various conventional deposition techniques have been developed for depositing a coating on a substrate. For example, chemical vapor deposition (CVD) produces a solid film on a substrate surface by thermal activation and surface reaction of gaseous reagents which contain the desired constituents of the film. Energy required to pyrolyze the reactants is supplied by heating the substrate. In order to achieve reasonable reaction rates, the substrate is heated to a relatively high temperature, in the range of about 500-2000xc2x0 F. These temperatures preclude application of the CVD process to heat sensitive substrate materials such as PC.
Another conventional deposition process, plasma enhanced chemical vapor deposition (PECVD), supplies energy to the reactants by an electrical discharge in a gas which forms a plasma in the deposition chamber. Generally the substrate is immersed in the plasma. The deposition rate, however, is usually low resulting in high process cost.
Physical vapor deposition (PVD) produces solid films by supplying thermal energy in the case of plasma spraying or thermal evaporation, or electrical energy in the case of reactive sputtering or electron beam evaporation. Plasma spray can obtain the rates needed at high temperatures but not at the temperatures where PC is stable. In addition, coatings tend to be highly stressed and porous. Reactive sputtering can be achieved at the desired substrate temperatures, but the deposition rates are too slow for a cost effective process. Thermal or electron beam evaporation can provide high deposition rates, but generally produce coatings with low adhesion due to low energy ions. Thermal and electron beam evaporation are also unable to provide simultaneous coating of two sides of a substrate.
Wire arc deposition is another coating method in which an arc is generated between a cathode and a wire of the material to be deposited. The wire is instantly melted, and the droplets are deposited onto the substrate. This method can deliver high coating rates, but the coatings typically have poor transparency and are porous. The films are also typically relatively thick, e.g. greater than the size of the droplets.
A conventional approach to protection of the PC material is to use an organic UV absorber within a silicone hardcoat. The problem with this approach, however, is that the organic UV absorbers degrade with time, thereby losing their ability to protect the PC.
Other potential UV filter coating materials include zinc oxide (ZnO) and titanium dioxide (TiO2). These materials, however, have been conventionally deposited by sputtering which typically has a very low rate.
In another conventional approach, U.S. Pat. No. 4,871,580 describes a conventional sputtering technique whereby a solid source material is sputtered into a plasma. However, this technique results in a much slower deposition rate, and thus may not be practical for commercial applications.
In view of the foregoing, it would be desirable to provide a method and apparatus for depositing a coating at a high rate on a substrate.
The present invention provides a method of coating a substrate, comprising the steps of generating a plasma which flows toward the substrate, evaporating a metallic reactant, and introducing the evaporated metallic reactant into the flowing plasma to project the metallic reactant onto the substrate.
The present invention also provides an apparatus for depositing a coating on a substrate comprising a first chamber, an anode and a cathode for generating an arc in the first chamber, a second chamber to house the substrate, the second chamber being in fluid communication with the first chamber, a pump for reducing the pressure in the second chamber to a value below the pressure in the first chamber such that a plasma generated by the anode and the cathode flows into the second chamber toward the substrate, and an evaporator which evaporates a metallic reactant into the flowing plasma.
The present invention also provides an evaporator comprising an evaporator chamber, a heating element in thermal contact with the evaporator chamber, a conduit mounted on the evaporator chamber to provide passage of a metal wire from a wire supply to said evaporator chamber, a motor adapted to feed the metal wire into said evaporator chamber; and a gas supply line coupled to the conduit.
The present invention also provides a method for coating first and second sides of a substrate comprising generating a first plasma which flows toward the first surface of the substrate, generating a second plasma which flows toward the second surface of the substrate, evaporating a metallic reactant introducing the evaporated metallic reactant into the first plasma to project the metallic reactant onto the first surface of the substrate, and introducing a second reactant into the second plasma to project the second reactant onto the second surface of the substrate.
Other features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings.